The present invention relates generally to the field of medical imaging and more specifically to the field of cardiac imaging by computed tomography. In particular, the present invention relates to the correction of motion artifacts, such as due to cardiac motion and to concentration variations of a contrast agent over time.
Computed tomography (CT) imaging systems measure the attenuation of X-ray beams passed through a patient from numerous angles. Based upon these measurements, a computer is able to reconstruct images of the portions of a patient""s body responsible for the radiation attenuation. As will be appreciated by those skilled in the art, these images are based upon separate examination of a series of angularly displaced projection images. It should be pointed out that a CT system produces data that represents the distribution of linear attenuation coefficients of the scanned object. This data is then reconstructed to produce an image, which is typically displayed on a cathode ray tube, and may be printed or reproduced on film. A virtual 3-D image may also be produced by a CT examination.
CT scanners operate by projecting fan shaped or cone shaped X-ray beams from an X-ray source that is collimated and passes through the object, such as a patient, that is then detected by a set of detector elements. The detector element produces a signal resulting from the attenuation of the X-ray beams, and the data are processed to produce signals that represent the line integrals of the attenuation coefficients of the object along the ray paths. A collection of these signals at a particular view angle is typically called a projection. By using reconstruction techniques, such as filtered back-projection, useful images are formulated from the projections. The locations of pathologies may then be located either automatically, such as by a computer assisted detection (CAD) algorithm or, more conventionally, by a trained radiologist. CT scanning provides certain advantages over other types of techniques in diagnosing disease particularly because it illustrates the accurate anatomical information about the body. Further, CT scans may help doctors distinguish between types of abnormalities more accurately.
CT imaging, when performed for moving structures such as the heart, has proven problematic due to the dynamic nature of the tissue, and it is only with the advent of faster CT scanners and multi-slice systems that imaging of certain types of structures, such as the heart in cardiac CT imaging, has proven feasible. However, even with these improved systems, the motion can produce image artifacts, such as streaking artifacts, which mask the anatomical structure. This type of image artifact may also be produced by the temporal variations in concentration of the contrast agents employed to enhance image acquisition, as of the cardiac chambers and vessels.
In particular, certain image artifacts, such as the streaking artifact, occur due to projection measurement inconsistency during image acquisition, such as resulting from cardiac motion or the time-varying concentration of the contrast agent In the case of cardiac motion, the change in shape of the scanned object results in projection measurement inconsistency. Similarly, the time-varying concentration of the contrast agent produces projection measurement inconsistency due to variations in the attenuation coefficients which result. Either or both of these effects is sufficient to produce image artifacts, such as rays or streaks which may obscure a clinically significant feature, such as a plaque or lesions. These image artifacts also reduce the diagnostic value of the CT images by failing to highlight areas of interest in which a contrast agent should flow.
Motion artifacts are typically addressed in CT imaging by either patient restraint to prevent motion, or by gating techniques. Patient restraint is obviously not effective in the case of cardiac CT imaging. Gating techniques operate by reconstructing the acquired images at the same point in a motion cycle, such as the cardiac cycle, thus minimizing motion artifacts in the reconstructed images. While gating is helpful in reducing motion artifacts, it requires a longer scan time because the technique employs only a subset of the acquired data for retrospective gating schemes. The longer scan time in turn requires increased amounts of breath holding and increased amounts of contrast agent. In addition, gating does not reduce artifacts related to time-varying concentrations of contrast agent and may even exacerbate this problem due to the reliance upon temporally staggered data sets.
Other methods of addressing motion, such as cardiac motion, include using two X-ray sources to obtain two sets of projection data identical in space but separated in time. The difference between the two sets of projection data allows the moving heart to be isolated and motion to be identified. This technique, however, requires hardware modification of the CT scanning system and does not reduce artifacts related to time-varying concentration of contrast agent.
Methods of reducing the artifacts caused by the time-varying concentration of the contrast agent, but not cardiac motion, also exist. For instance, methods of projection interpolation have been proposed which are simple to implement but have only limited effectiveness in reducing the image artifacts. Other methods have been proposed which involve multiple computational steps, allowing errors to accumulate throughout the calculations. These accumulated errors may themselves distort the final image.
As noted, the existing techniques generally fail to address the potential causes of image artifacts, such as streaking artifacts. Likewise, many of the techniques are either largely ineffective at artifact reduction in general or themselves distort the final image. An effective method of reducing such image artifacts, while maintaining or improving the clinical diagnostic value of the final image, is therefore needed.
The present technique provides a novel technique to minimize or eliminate motion-related artifacts produced when imaging dynamic tissue, such as cardiac tissue. Particularly, the technique provides for a method and system for averaging projection values corresponding to the projection of a region of interest in the object at each view angle. The technique thereby allows motion related artifacts, such as streaking, to be minimized or eliminated.
In accordance with one aspect of the technique, a method is provided for imaging a dynamic region of interest using a multi-row CT imaging system. The technique provides for segmenting a region of interest. Two or more successive projections from a projection data set are selected. Each of the two or more successive projections is acquired at the same view angle and axial position, and a portion of data corresponding to a projection of the region of interest upon a detector is identified. The portion of data of each of the two or more successive projections is averaged to produce an averaged portion. The portion of data of each of the two or more successive projections is replaced with the averaged portion to create two or more respective averaged projections. The two or more averaged projections are combined with the remainder of the projection data set to produce a formatted data set. An image is reconstructed from the formatted data set.
In accordance with a further aspect of the technique, a CT cardiac image analysis system is provided which includes a computer system capable of being operably coupled to at least one of a CT cardiac image acquisition system or CT image storage system. The computer system is configured to segment a region of interest. In addition, the computer system is configured to select two or more successive projections from a projection data set. Each of the two or more successive projections are acquired at the same view angle and axial position and a portion of data corresponding to a projection of the region of interest upon a detector is identified. The computer system is also configured to average the portion of data of each of the two or more successive projections to produce an averaged portion and to replace the portion of data of each of the two or more successive projections with the averaged portion to create two or more respective averaged projections. The computer system is also configured to combine the two or more averaged projections with the remainder of the projection data set to produce a formatted data set and to reconstruct an image from the formatted data set. The image analysis system also includes an operator workstation operably coupled to the computer system configured to display the image.
In accordance with another aspect of the technique, a CT cardiac image analysis system is provided. The CT cardiac imaging system includes a means for acquiring a set of CT projection images. The CT cardiac imaging system also includes a means processing the set of CT image projections such that motion artifacts are minimized in a reconstructed image.
In accordance with a further aspect of the technique, a tangible medium is provided which includes a routine for segmenting a region of interest. The tangible medium also includes a routine for selecting two or more successive projections from a projection data set. Each of the two or more successive projections is acquired at the same view angle and axial position, and a portion of data corresponding to a projection of the region of interest upon a detector is identified. In addition, the tangible medium includes a routine for averaging the portion of data of each of the two or more successive projections to produce an averaged portion. A routine is also included for replacing the portion of data of each of the two or more successive projections with the averaged portion to create two or more respective averaged projections and for combining the two or more averaged projections with the remainder of the projection data set to produce a formatted data set A routine is included for reconstructing an image from the formatted data set.